For the treatment of organically-polluted water containing soluble organics as primary pollutants, a variety of biological treatment processes are known to artificially and efficiently perform self-purification which takes place in the natural world. Such biological treatment processes have found wide-spread utility for their relatively economical treatment cost, and especially, the activated sludge process making use of aerobic bacteria is practiced in many fields.
As the mechanism of purification treatment in the above-mentioned activated sludge process, soluble organic pollutants indicated by “BOD” are promptly degraded by bacteria (aerobic bacteria), the resulting, proliferated bacteria are subjected to predation by protozoa, solid organics are put together with the thus-proliferated protozoa as an adhesive to form large flocs, and then, the flocs are subjected to solid-liquid separation in a settling tank so that organically-polluted water is purified to obtain treated water. In the activated sludge process, the living activities of aerobic bacteria and protozoa, therefore, have to be performed under good conditions. The flow through a treatment tank (aeration tank) is, therefore, controlled uniform such that the treatment tank always remains under constant aerobic conditions.
As described above, the activated sludge process can subject soluble organics in organically-polluted water to biodegradation an aeration tank, but no degradation of solid organics takes place so that the solid organics are subjected to solid-liquid separation in a settling tank arranged down stream of the aeration tank. In other words, biodegradation is not the entirety of the purification treatment by the activated sludge process, and as a matter of fact, its essence is to achieve the solid-liquid separation of suspended materials (SS; the abbreviation “SS” as used herein stands for “solid organics”) from organically-polluted water.
Because the activated sludge process can be considered to have as its essential object the removal of organics from organically-polluted water by solid-liquid separation as mentioned above, a great deal of sludge is naturally produced through the solid-liquid separation. In the activated sludge process, therefore, a fraction of the activated sludge after the solid-liquid separation is returned as much as needed for the subsequent purification treatment of organically-polluted water as recycled sludge to the aeration tank, and is used in the subsequent biological treatment. The remaining activated sludge is eliminated as surplus sludge. The treatment and disposal of huge surplus sludge, therefore, still remains as a serious problem in the activated sludge process.
With a view to overcoming the above-described problem, a variety of attempts have been made in efforts to efficiently treat surplus sludge or to reduce the amount of surplus sludge to be produced. No treatment method is, however, known to eliminate surplus sludge (in other words, to reduce the production of surplus sludge to zero) and moreover, to be free from any problem of secondary pollution by the use of a chemical or the like while enjoying an economical advantage. Especially in recent years where the protection of the global environment has been strongly urged, this problem of surplus sludge treatment is serious, leading to a keen desire for the development of a process of the purification treatment of organically-polluted water without such a problem.
In addition to the above-described problem of surplus sludge treatment, the activated sludge process is accompanied by another problem as will be described next. As the BOD pollutant load that can be treated in an aeration tank by the activated sludge process is limited, the treatment of organically-polluted water is conducted by aerating the aeration tank to always maintain it under constant aerobic conditions and controlling the residence time of the organically-polluted water such that the concentration of soluble organic pollutants in organically-polluted water within the aeration tank as expressed in terms of “BOD” drops to a preferred range. The treatment of organically-polluted water containing soluble organics at high concentration, therefore, requires to scale up the treatment facilities.
In the case of water organically polluted at high concentration, the organically-polluted water which is flowing in under goes substantial fluctuations in BOD and SS concentration, thereby making it difficult to control the interior of the aeration tank under such conditions that its purification treatment can be performed always well. The activated sludge treatment of water organically polluted at high concentration, therefore, requires to arrange an adjustment tank in an upstream stage. As has been described above, it is difficult for the conventional activated sludge process to efficiently treat water organically polluted at high concentration such as sewage or wastewater from a food-related factory, which contains organics as primary pollutants, specifically contains soluble organics and solid organics at high concentrations (for example, which has 100 mg/L or higher BOD and 100 mg/L or higher SS).
In addition to the activated sludge process, the catalytic oxidation process is also known. Compared with the activated sludge process, however, this catalytic oxidation process is adopted to extremely limited extent only, and is used merely as a final finishing step in the activated sludge process or as a pretreatment step at waterworks. At the beginning, the tricking filter process, a catalytic oxidation process, was used. This process certainly makes it possible to reduce the production of sludge by 10 to 20% or so in comparison with the activated sludge process, and therefore, is not free from the production of sludge. It also involves other problems such as the production of an offensive odor and the growth of filter flies, and accordingly, it is scarcely performed these days. As its replacements, catalytic oxidation processes making use of rotary disks, honeycomb tubes or various other plastic materials have been also proposed. They are also practiced to limited extents only.
Recently, an attempt has been made to improve the efficiency of treatment by adding a carrier into an activated sludge tank to make partial use of catalytic oxidation in combination. Even in such a case, however, the production of sludge can be reduced by as little as 5 to 20% or so in comparison with the standard activated sludge process.
As equipment for decreasing the amount of surplus sludge produced by the activated sludge process, it is practiced to arrange a digestion tank. This digestion tank is a piece of equipment that heats sludge at 37° C. under anaerobic conditions and allows the sludge to stay there for 30 days. Even with this equipment, however, the decrease in the amount of the charged sludge is around 40% or so at the maximum.
According to the activated sludge process which is almost exclusively used in the current treatment of organically-polluted water, the aeration tank is maintained under constant aerobic conditions while the digestion tank is kept under constant anaerobic conditions, as mentioned above. Therefore, the degradation of sludge (solid organics) is 5% or so at the maximum in the former and is 40% or so at the maximum even in the latter.
The present inventor proposed a separation material for the purification of polluted water (see JP-B-08-017901). Under the current circumstances of the biological treatment of organically-polluted water as described above, the separation material enables totally novel purification treatment of organically-polluted water by making positive use of self-purification which takes place in the natural world. The separation material is formed of aggregate, which in turn comprises a plurality of crushed stones of 1 to 3 cm in average diameter. As techniques making use of the aggregate (aggregative purifying material), the present inventor also proposed a purification process for more efficiently performing purification of organically-polluted water (see JP-A-08-332497) and a purification process of polluted water by the use of an aggregative purifying material composed of hollow members having numerous openings in their surfaces (see JP-A-08-108191) . According to these techniques, the separation of a solid suspended in polluted water can be effectively performed, and the purification of still finer solid pollutants or the like can be conducted by causing organically-polluted water to pass through a purification zone in which an aggregative purifying material such as that described above is packed and arranged in stacked plural layers.